In an optical communication system, an optical transmission module converts an electrical signal to an optical signal, and its performance affects the system performance directly. Therefore, an optical transmission module is important in the system. Conventionally, an optical transmission module with analog adjustment is shown in FIG. 1.
In FIG. 1, the optical transmission module is mainly consisted of four parts: a laser 11, which performs electro-optical conversion; a laser driver 12, which makes impedance match, drives the laser and sets parameters; an automatic power control (APC) circuit 13, which makes power of the transmitted signal stable; and an automatic temperature control (ATC) circuit 14, which makes temperature of the laser stable to guarantee a stable optical wavelength.
Input signals of an optical transmission module include a differential clock signal and a differential data signal, which are inputted to the laser driver 12, the laser shutdown signal, which is inputted to the APC circuit 13. The signals for reporting upward performance of an optical transmission module, include the laser refrigerating current outputted from ATC circuit 14; the average power of a transmitted optical signal outputted from APC circuit 13, and the laser bias current.
Parameters of the optical transmission module are adjusted mainly through potentiometers. Extinction ratio and cross point of the laser is adjusted by the potentiometers R1 and R2 through the laser driver 12. The optical wavelength trimming signal of the optical transmission module is adjusted by the potentiometer R3 through ATC circuit 14. The optical power of the optical transmission module is adjusted by the potentiometer R4 through APC circuit 13.
There are also other additional circuits, such as a laser shutdown circuit, a power supply etc. Since they are no relation with the invention, so they have not been drawn here.
Advantages of the optical transmission module with analog adjustment are mature technically, and are popularly used by communication device vendors; but there are disadvantages:
(1) Accuracy and stability of parameters of the optical transmitter depends on accuracy and stability of the potentiometers, and a potentiometer changes its resistance by changing the slider position, which is a mechanical device. Since contacts of a mechanical potentiometer have problems of contact fault, temperature drift and contact moving etc., drift of potentiometer parameters is unavoidable.
(2) adjustment of a potentiometer is complicated so that it is difficult for an optical transmission module to be implemented automatic production and production cost will be higher during manufacture. In addition, performance of an optical transmission module depends on skills of workers, so it is difficult to guarantee performance consistency of products.
(3) It is difficult to create a product database for parameters of adjustment and testing of optical transmission module products. The product database is very important for ameliorating performance of optical transmission module.
(4) During application, parameters of an optical transmission module will be drift along with increasing of service time. It is necessary to adjust parameters of an optical transmitter without interrupting its service, but this is impossible for a device adjusted by potentiometers.
In recent years, along with the development of digital technique many analog techniques are replaced by digital techniques. The invention intends to implement digital adjustment of parameters of an optical transmission module.